Climatic conditions are important in the design of air separation apparatuses and, more generally, in cryogenic apparatuses. More particularly, the cooling water of the refrigerators of the various compression stages of the air compressor can vary according to the climate and even between day and night, significantly in some countries, so that, in these countries, fluctuations in the temperature of the water of the order of 15.degree. C. can be recorded.
These variations are currently resolved by installing, at the outlet of the final refrigerator, a refrigerating unit supplying the additional cooling power which the water was not capable of giving.
Refrigerating units have the drawback of being an expensive investment and of using at least one rotating machine, which is unreliable and heavily consumes energy.
U.S. Pat. No. 4,375,367 describes a system in which a flow of air to be distilled is cooled, before being purified, by recycling the air produced by the purification system. Nevertheless, the use of a refrigerating unit is indispensable in this case.
EP-A-0,624,765A discloses a system which makes it possible to substitute for the refrigerating unit a system for heat exchange with a flow of pressurized fluid originating from the air separation installation. The use of a cycle fluid for cooling the air upstream of the purification system is not described.
This patent application also does not disclose an installation in which the air is precooled in an auxiliary exchanger with only one other fluid.
J-A-54,103,777 describes the use of a flow of nitrogen originating from a distillation column for cooling the sir to be purified.
EP-A-0,505,812 discloses that a flow of air to be purified can be cooled with a flow of purified air, before the latter has its pressure reduced.